Polygon digitalization

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Polygon digitalization

bertGIS
Hello.

My firm passed under QGis one year ago, and we encounter a problem with the use of QGis for our botanists.
Indeed they will be confronted with the need of digitizing hundreds of habitats polygon with correct joins.

Some have tested under QGis Lisboa and the result is discussable because, as you can see on the attached image, the created polygons (with the option "avoid intersections" and "topological editing" checked) shows some garbage. Here I have moved one of them and we see the results.



Reading the former messages on this subject, I have tested the options of digitizing polilines with 'traceedit' and poligonize with 'poligonizer', and the GRASS vector layer. But it is not practical for our experts.

Can you help me about that ?

Thanks
Cheers
Bert
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Re: Polygon digitalization

Nick Hopton
Just checking Bert, but have you seen this?

http://confound.me.uk/maps/ppv4.pdf

This is the way I work, I find it quicker than tracing polygons and it also separates the job of digitising from the job of entering data (which I find convenient).

The only other way I can think of would be to start off with a large polygon and slice it up into smaller ones using the split features tool. Not tested here and I don't think this method will let you cut holes in polygons.

Nick.
SBL
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Re: Polygon digitalization

SBL
In reply to this post by bertGIS
Hi Bert

I know quite a few people (including me) who ran into the same, pretty anoying, problem.

And in general it is a known issue. See: http://hub.qgis.org/issues/2921
Did I understand you correctly that you already tried the workaround Marco suggests at the end of the ticket? This would indicate that activating topological editing is not enough (at least not in QGIS 1.8)...
Did you also try activating snapping on lines and nodes as well?

From my personal point of view this issue is a mayor obstacle of using QGIS for digitising (though the digitising functions otherwise are really good and handy). I would really love to see a propper fix for that and that digitizing can not silently end up in incorrect results.

At least you could clean your already digitised and incorrect data using GRASS and the topological vector format there...

Cheers
Stefan
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Re: Polygon digitalization

Marco Hugentobler-4
Hi Stefan

Did you try a developer version (e.g. nightly build)? I think my
workaround does solve the problem for most usages. It does not solve the
problem only if you do avoid intersection to background layers and those
layers are not editable.

>I would really love to see a propper fix for that and that
>digitizing can not silently end up in incorrect results.

Then you might consider contributing to the geos project
(http://trac.osgeo.org/geos/).

Regards,
Marco

Am 19.03.2013 21:11, schrieb SBL:

> Hi Bert
>
> I know quite a few people (including me) who ran into the same, pretty
> anoying, problem.
>
> And in general it is a known issue. See: http://hub.qgis.org/issues/2921
> Did I understand you correctly that you already tried the workaround Marco
> suggests at the end of the ticket? This would indicate that activating
> topological editing is not enough (at least not in QGIS 1.8)...
> Did you also try activating snapping on lines and nodes as well?
>
>  From my personal point of view this issue is a mayor obstacle of using QGIS
> for digitising (though the digitising functions otherwise are really good
> and handy). I would really love to see a propper fix for that and that
> digitizing can not silently end up in incorrect results.
>
> At least you could clean your already digitised and incorrect data using
> GRASS and the topological vector format there...
>
> Cheers
> Stefan
>
>
>
> --
> View this message in context: http://osgeo-org.1560.n6.nabble.com/Polygon-digitalization-tp5040680p5041455.html
> Sent from the Quantum GIS - User mailing list archive at Nabble.com.
> _______________________________________________
> Qgis-user mailing list
> [hidden email]
> http://lists.osgeo.org/mailman/listinfo/qgis-user


--
Dr. Marco Hugentobler
Sourcepole -  Linux & Open Source Solutions
Weberstrasse 5, CH-8004 Zürich, Switzerland
[hidden email] http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee

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Re: Polygon digitalization

SBL
Hi Marco

I had not tried for a while and just followed the discussion and activity regarding this issue.
Tonight I tried again both on QGIS 1.8 and 1.9 through OSGeo4w on Windows 7 (64 bit).
I updated QGIS-dev (to Code revision 32de07d) before testing and can still recreate the behavior, regardless if "Enable topological editing" is activated or not. I also activated "Enable snapping on intersection", set the snapping tolerance to 0.001 meter for both vertex and segments and digitized at at 1:1000 scale. Not successfull for 1.8 and 1.9-Master. I have to admit that I created rather artificial polygons for testing, but land cover and vegetation can have quite complex forms as well...
When I tested I had only one layer in the project, the one I was editing...

I can try on Ubuntu tomorrow...

Best regards
Stefan
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Re: Polygon digitalization

Gerardo Jimenez
I usually prefer to set the tolerance at 15 pixels not

" the snapping tolerance to 0.001 meter for both vertex and segments and
digitized at at 1:1000 scale"

(it looks to me too little tolerance)

Give it a try (I am working with Lisboa in  win 7 32 and 64)

Gerardo Jiménez Delgado
Instituto de Investigaciones Antropológicas
Universidad Nacional Autónoma de México
Ciudad Universitaria s/n
Coyoacan 04510 Mexico City
Mexico 56 22 95 16

> Date: Tue, 19 Mar 2013 15:34:34 -0700

> From: [hidden email]
> To: [hidden email]
> Subject: Re: [Qgis-user] Polygon digitalization
>
> Hi Marco
>
> I had not tried for a while and just followed the discussion and activity
> regarding this issue.
> Tonight I tried again both on QGIS 1.8 and 1.9 through OSGeo4w on Windows 7
> (64 bit).
> I updated QGIS-dev (to Code revision 32de07d) before testing and can still
> recreate the behavior, regardless if "Enable topological editing" is
> activated or not. I also activated "Enable snapping on intersection", set
> the snapping tolerance to 0.001 meter for both vertex and segments and
> digitized at at 1:1000 scale. Not successfull for 1.8 and 1.9-Master. I have
> to admit that I created rather artificial polygons for testing, but land
> cover and vegetation can have quite complex forms as well...
> When I tested I had only one layer in the project, the one I was editing...
>
> I can try on Ubuntu tomorrow...
>
> Best regards
> Stefan
>
>
>
> --
> View this message in context: http://osgeo-org.1560.n6.nabble.com/Polygon-digitalization-tp5040680p5041490.html
> Sent from the Quantum GIS - User mailing list archive at Nabble.com.
> _______________________________________________
> Qgis-user mailing list
> [hidden email]
> http://lists.osgeo.org/mailman/listinfo/qgis-user

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Re: Polygon digitalization

SBL
Hi Gerardo

Now I tried with 15 pixels snapping threshold on QGIS 1.9-Master. Maybe the problem occures less often, but it still occures...

Cheers
Stefan